This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-072854, filed Mar. 15, 2000; and No. 2001-048579, filed Feb. 23, 2001, the entire contents of which are incorporated herein by reference.
The present invention relates to an ultrasonic transducer and ultrasonic transducer system for use in harmonic imaging ultrasonic diagnosis, and particularly to an ultrasonic transducer which transmits a fundamental ultrasound having a center frequency f0, and detects the reflected ultrasound having a center frequency nf0 (n: integer of 2 or more), generated by the propagation of the fundamental ultrasound.
In recent years, harmonic imaging ultrasonic diagnosis has attracted attention. A diagnosis method is roughly classified into a contrast harmonic imaging using a contrast medium, and tissue harmonic imaging of detecting the non-linearity of an elastic property of a living tissue, and displaying the non-linearity in an image. The situation is described in detail in xe2x80x9cspecial issue on electronics clinical medicine ultrasoundxe2x80x94Latest Ultrasoundxe2x80x94: distributed text of 1999 academic lecture by the Japan Society of Ultrasound in Medicinexe2x80x9d.
The tissue harmonic imaging is a technique of transmitting an ultrasonic pulse having a center frequency f0 to a living tissue without using the ultrasonic contrast medium, extracting a high order harmonic component nf0 (n being an integer of 2 or more) included in a returned echo signal, and displaying a relation between an amplitude of the component and an echo signal receiving time in a tomographic image to obtain a diagnosis image.
For an in vitro purpose, a diagnosis apparatus with the aforementioned function mounted thereon is already on the market. In the tissue harmonic imaging diagnosis method, heart structures such as a left chamber wall can be relatively clearly observed, even in an overweight person, an aged person or a person who smokes, whose echo image has been frequently blurred because of mixed noise.
The ultrasonic diagnosis method is at present used only for the in vitro purpose, and a second order high harmonic wave (n=2), that is, the ultrasound having a center frequency of 2f0 is used. In a conventional ultrasonic transducer, transmission of the ultrasound having a center frequency f0 and reception of the ultrasound having a center frequency 2f0 are performed by the same ultrasonic vibrator. Therefore, the ultrasonic vibrator used needs to have a remarkably broad band.
Moreover, to further enhance the resolution, utilization of a third order harmonic signal is expected, but an ultra-broad band ultrasonic vibrator which can detect an ultrasound having a center frequency of 3f0, that is, a third order harmonic signal has not been realized yet.
It is usually said that the sensitivity of a second order harmonic signal is deteriorated by 15 to 20 dB, and a third order harmonic signal is further deteriorated by 15 to 20 dB as compared with the fundamental frequency signal. Therefore, the aforementioned sensitivity deterioration with the broadened band disadvantageously causes further deterioration of the diagnosis image.
Furthermore, since the transmission of the ultrasound having the center frequency f0 and the reception of the ultrasound having the center frequency 2f0 are performed by the same ultrasonic vibrator, a fundamental wave and various unnecessary vibrations are unavoidably superimposed onto a received ultrasonic signal.
To improve such disadvantages, Jpn. Pat. Appln. KOKAI Publication No. 11-155863 discloses an ultrasonic transducer which has a transmitting piezoelectric resonator and receiving piezoelectric resonator in one case which can efficiently receive the high order harmonic component. A constitution of the ultrasonic transducer is shown in FIG. 31.
As shown in FIG. 31, an ultrasonic transducer 1000 has a transmitting piezoelectric resonator 1002, and a receiving polymer piezoelectric resonator 1004 disposed in front of the transmitting piezoelectric resonator. The receiving polymer piezoelectric resonator 1004 and transmitting piezoelectric resonator 1002 are layered and disposed via an acoustic matching layer 1006.
Front electrodes of the transmitting piezoelectric resonator 1002 and receiving polymer piezoelectric resonator 1004 are both connected to a grounding lead wire 1008 and are kept at a ground potential. A back-side electrode of the transmitting piezoelectric resonator 1002 is connected to a transmitting shielding wire 1010, and a drive signal is supplied via the wire. A back-side electrode of the receiving polymer piezoelectric resonator 1004 is connected to a receiving shielding wire 1012, and a received signal is extracted via the wire.
The transmitting piezoelectric resonator 1002 has a resonant frequency or an antiresonant frequency which agrees with a resonant frequency of the ultrasonic contrast medium or a frequency having a specific relation with respect to the ultrasonic contrast medium. On the other hand, the receiving polymer piezoelectric resonator 1004 is a non-resonating piezoelectric resonator, and can receive even the high order harmonic component generated based on the nonlinear behavior of the ultrasonic contrast medium.
Since the acoustic matching layer 1006 is disposed between the transmitting piezoelectric resonator 1002 and the receiving polymer piezoelectric resonator 1004 in the ultrasonic transducer 1000, only a portion with the ultrasonic contrast medium present therein, such as a blood vessel in a human body and a cancer tissue with capillary concentrated on a peripheral portion thereof, can be depicted more clearly than other portions.
Since the ultrasonic transducer 1000 has separate transmitting and receiving piezoelectric resonators, the band is easily broadened, and properties suitable for harmonic imaging are expected to be displayed, as compared with the conventional ultrasonic transducer for general use for performing transmission/reception with the single piezoelectric resonator.
However, in the conventional ultrasonic transducer shown in FIG. 31, the transmitting and receiving ultrasonic vibrators are superposed and disposed. Therefore, when a transmitted ultrasonic wave is passed through the receiving ultrasonic vibrator, the ultrasonic wave excites the receiving ultrasonic vibrator and is modulated by the vibration. As a result, undesired vibration of the resonant frequency of a receiving ultrasonic vibrator film is mixed in with the transmitted ultrasonic wave. This means that it is impossible to judge whether the signal detected by the receiving ultrasonic vibrator is the high order harmonic signal from the ultrasonic contrast medium or the signal mixed during transmission. Therefore, the mixture of the undesired vibration causes a large deterioration of the resolution.
Moreover, for use in a so-called tissue harmonic imaging (THI) for detecting a nonlinear ultrasonic wave generated with propagation of the fundamental ultrasonic wave in the living tissue, the high order harmonic wave needs to be securely selected and detected, because a sound pressure level of the nonlinear ultrasonic wave generated with the propagation of the fundamental ultrasound in the living tissue is as small as about xe2x88x9220 dB, as is well known. However, in the conventional ultrasonic transducer shown in FIG. 31, since the receiving ultrasonic vibrator has a non-resonating broad-band property, also for the received signal, the high order harmonic signal level is xe2x88x9220 dB lower with respect to the fundamental wave and such a situation is unchanged.
An object of the present invention is to provide a technique of an ultrasonic transducer which has a transmitting piezoelectric resonator and receiving piezoelectric resonator contained in the same case, but which can detect a harmonic signal with a high sensitivity without being adversely affected by resolution deterioration caused by residual vibration.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.